MOFGA Pest Report 2004 #
24 May 2004

(View List Of All 2004 Pest Reports)

NOTE ON PICTURES

Insect pests: http://www.nysaes.cornell.edu/ent/factsheets/
Plant diseases: http://vegetablemdonline.ppath.cornell.edu/Home.htm

FLEA BEETLES IN BRASSICAS

Other species of flea beetles attack other crop families: solanaceous crops such as eggplant, potato, and tomato, or sweet corn. These beetles may look very similar to those found on Brassicas, but if they are feeding on a different crop group, they are almost certainly a different species of flea beetle. Keep this in mind when planning crop rotations Feeding damage and crop preference

Flea beetle adults feed on leaves and stems, resulting in numerous small holes, or 'shot-holes'. Heavy feeding can kill plants, especially seedlings, and moderate damage can reduce plant size, delay maturity, reduce yield, or render crops unmarketable. Flea beetle larvae feed on roots. The impact of their feeding damage on crop yield is not well understood.

Flea beetles show differences in preference and feeding behavior among the Brassica species. Most of the Brassica vegetable crops that are of European origin (cabbage, broccoli, cauliflower, Brussels sprouts, collards, kale, and kohlrabi) are variations of the same species, Brassica oleracea. The cotyledons of these crops are very attractive to flea beetles, but as plants develop the leaves become more waxy. The waxy surface is more difficult for beetles to grasp and feed; as a result, beetles feed mostly at the leaf margins once true leaves have developed. This is especially true for cabbage. While waxiness varies among these crops, in general, once seedlings are past the two or three leaf stage, flea beetles tend to be less of a pest problem. Many Brassica greens and also Brassica root crops - belong to different Brassicas species that are more preferred by flea beetles. These include Brassica rapa (Pac choi, Choi Sum, Chinese cabbage, tatsoi, mizuna, komatsuna, turnip), B. juncea (mustards), and B. napus (red Russian kale, rutabaga). Other highly attractive Brassicas including Raphanus sativus (radish, daikon) and Eruca vesicaria (arugula). Most farmers who produce Brassica greens are well aware of the preference that flea beetles have for these species compared to the Brassica oleracea. These species all have non-waxy leaves that are easier to grasp and feed. There may be other differences in plant chemistry that play a role. Larvae survive better on some of these species. In Brassica greens, beetles feed on the whole surface of the leaf, and will continue feeding from the seedling stage until harvest. Holes that are made to small leaves expand as the leaf grows. Control is needed throughout crop growth. While some markets are more demanding than others, many markets will not accept greens with even slightly shotholed appearance.

Flea beetle life cycle

In the autumn, adult flea beetles move into shrubby or wooded areas near fields where they have been feeding. At UMass we have sampled overwintering beetles in different types of habitats in or next to fields. We found the highest numbers in shrubby borders, somewhat fewer in the woods, and virtually none in more open grassy areas near the edge of the field. Adults leave overwintering sites in April and early May, to feed, mate, and lay eggs. In 2003 we dissected beetles that were collected from the field on a weekly basis, to determine when female beetles developed eggs. Eggs were found in female beetles from mid May until the end of August.

Eggs are laid in the soil near host plants, and larvae feed on root hairs and roots. After a pupal stage, new adults emerge from the soil. The rate of larval development is temperature-driven, so cooler soils will result in slower growth and delayed emergence of the summer adult generation. It requires 456 Degree days (with a base temperature of 51.8°F, or 11°C) for development from egg to adult (Kinoshita 1979). Depending on temperature, the new generation of adults will begin to emerge in mid to late July. Based on what we found in 2003, we believe that at least some of these summer beetles produce eggs. These offspring probably emerge as adults during the fall.

Newly emerged beetles appear to have a strong propensity to feed. In cage studies of field-collected beetles, adults collected in early August fed at a higher rate (ie, the same number of beetles produced more holes per plant in 24 hours) than those collected in spring. In late August and early September, feeding rates declined greatly. At that time, beetles are probably leaving the field for overwintering sites.

Management strategies

Basically, strategies for flea beetles could be summarized as one of five options:

1) Escape them
2) Starve them
3) Kill them
4) Shut them out
5) Combine as many of these as possible.

Here are more details:

Crop rotation

To reduce and delay flea beetle invasion of spring crops, move them as far away from the fields that were used for fall Brassica crops as possible. Barriers such as woods, roads, waterways, etc. help slow movement from overwintering sites to the new spring field. We do not know enough about beetle flight patterns to answer the question "how far is far enough?" We do know that any rotation is better than none, and the farther the better.

Delayed planting

If no Brassica crops are planted until mid to late July, this will stop the reproductive cycle because overwintered beetles will have no where to feed or reproduce (except on Brassica weeds). This strategy can be very effective in bringing the numbers down, depending on the weed population. Some mesclun growers use only non-Brassica greens in their mix until late summer. Depending on your markets, this strategy may require serious adjustments to the farm's production and marketing plan - and may not be feasible. It is hard to imagine a successful farmstand or CSA farm with no Brassicas until fall.

Separate early and late crops

Let's assume that emergence of the summer generation of beetles from a spring crop begins in mid July. If there are young Brassicas close by, they will be heavily damaged. However, if fall Brassicas are seeded into an isolated, rotated field, beetle numbers will be low and the crop will suffer much less damage.

Provide crops with good growing conditions

Research studies have shown that well-fertilized plants growing in good soil with adequate water are attacked less than plants that are wilted, poorly fed, or growing in compacted soil. While this may not overcome a large flea beetle population, it can make a difference in plant survival, growth and quality.

Use row covers

One of the best ways to protect Brassica crops from flea beetles is to place a floating row cover over the bed or row. In every pesticide trial conducted at UMass, the cleanest, highest quality greens are always those under row cover. For growers with relatively small (< 1/2 acre) plantings row covers can be practical and cost-effective; however, they are a management-intensive system.

It is critical to seal the edges immediately after seeding, because Brassica seeds germinate quickly and beetles rapidly find the cotyledons. Flea beetles can fit through extremely tiny cracks. Edges of the cover must be sealed on all sides using soil, plastic bags filled with soil, or some other method. Occasionally growers leave a few uncovered plants at the end of the row and don't seal the ends of the row cover. The open crop attracts beetles to the bed, where they may crawl underneath the cover. Wide row covers have the advantage of protecting a larger area relative to the length of the edges. However, if there are raised beds, the furrow may have a large gap that allows entry. Make sure to weight the ends in the furrow.

The weight of the row cover fabric should be considered. Heavier covers are more durable, an important factor, given the cost of row cover. They transmit less light, and provide more heating and more cold protection. Lower light transmission increases tenderness and length of the greens, which can be desirable. However, if heavier covers are used in midsummer, yield may be reduced. Non-heating, reinforced covers may be desirable for midsummer plantings. Weed control is another major issue with row covers. Preparing a stale seedbed before seeding, using flaming, cultivation or herbicide, will help delay weed emergence.

We have also observed a compost mulch being used with success for intensively planted beds of greens. Even when weed control techniques are used prior to planting, covers may need to be removed for cultivation or hand hoeing. To minimize beetle entry, replace covers the same day.

Insecticides

Flea beetles are especially difficult because there have been no effective approved insecticides and many growers depend on succession-planted greens as a key part of their crop mix. However, insecticide trials at UMass and the CT Agricultural Experiment Station have provide some promising results.

For the past three years we have conducted insecticide trials at the UMass Research farm with the goal of identifying low-risk and organic insecticides that will suppress or control flea beetles. We conducted these trials in 2002 and 2003, using Komatsuna, a B. rapa species that can be harvested for bunched greens five weeks after seeding. We used small replicated plots, with weekly spray applications. Flea beetle populations were high. Carbary (Sevin) was used as standard conventional insecticide, for comparison. Leaf damage (number of holes per plant) and plant weight at harvest and beetle counts were used to compare treatments.

Carbaryl and row cover treatments had the lowest damage, and plant weights were signficantly higher than the untreated controls for carbaryl in both years and for row cover in 2002. Kaolin (Surround, 1/2 lb/gal.) and pyrethrin (Pyganic EC 5.0, 16 oz/A) treatments had as much damage as the untreated control.

Spinosad was tested in both Spintor (a conventional formulation, 5 oz/A) and the Entrust (an organically approved formulation, 1.5 oz/A). Both of these reduced leaf damage, although not as effectively as Sevin or row cover. This result was consistent in both years, although statistically only the final harvest in 2002 was not different from the control . The level of protection provided by spinosad, though far from perfect, is an encouraging result for organic growers. Both are labeled for use in Brassica crops, although flea beetle is not listed as a target pest on the label. Thus it is legal to use Entrust in organic Brassica crops, and growers who use it can anticipate suppression of flea beetles and reduction of feeding damage. There is a limit of three applications allowed in a 30 day period. Capsaicin (Hot Pepper Wax, 8 oz/gal.) was tested in 2002 and reduced leaf damage as much as carbaryl and spinosad.

Because this product is currently not allowed by the NOP we did not test this again in 2003. Provado was tested in 2001, and did not provide suppression in that trial. Similar experiments were conducted by Dr. Kim Stoner at the Connecticut Agricultural Experiment Station, comparing several organic treatments. Spinosad, pyganic and hot pepper wax all showed significantly less damage than the control. Spinosad, both Entrust and Spintor formulations, gave the highest levels of control. Pyganic was intermediate. Neem (Aza-direct) and insecticidal soap were no different from untreated plots.

Reference:

Kinoshita G.B., H.J. Svec, C.R. Harris and F.L. McEwan. 1979. Biology of the crucifer flea beetle, Phyllotreta cruciferae (Coleoptera: Chrysomelidae), in southwestern Ontario. The Canadian Entomologist 111:1395-1407

(modified from a report in the Mass. Vegetable News by Ruth Hazzard, Caryn Andersen and Roy Van Driesche, Dept of Entomology and Frank Mangan, Dept of Plant and Soil Science, University of Massachusetts)

ONION MAGGOT (Delia antiqua)

The greatest damage is caused by the first-generation larvae that attacks young seedlings in the early spring. They move up rows to new plants as they devour the young seedlings quite quickly and need more food. The first symptom seen in very young seedlings is a slight wilting of the plant. Later they will simply disappear. Plants attacked at later seedling stages will turn a grayish yellow and wilt and will later detach at the ground level as the maggot consumes everything below ground. Onion plants attacked later are rarely killed and fewer plants are attacked as the maggot does not move to new bulbs. However, plants attacked at the later stage are likely to have misshapen bulbs and usually are attacked by fungi and rot.

The onion maggot overwinters as a pupa in the top five or so inches of soil. When spring soil temperature rises above about 40 degrees the overwintering pupae begin to develop and depending on the weather in a number of weeks the adult onion maggot flies will emerge and fly around and mate. Later they will search for onions to lay eggs around (late April this year here in central Maine). Eggs are laid around the base of onion plants and the resulting larvae crawl down to start feeding on the roots. There can be three generations in a season.

Adults of later generations disperse very little from onion fields so crop rotation is a very important tool for avoiding infestations. Good sanitation is very important because cull onions will attract flies that will lay eggs that result in overwintering pupa. Damaged onions are the preferred site for egg laying so avoid mechanical damage to onions and dispose of any damaged onions rather than leaving them laying around the field. Discing onion fields in the early fall when the flies are still active actually makes things worse because it increases the number of sites for egg laying. Ash and diatomaceous earth around the base of onion seedlings may deter egg laying and maggot survival.

CUTWORMS

HEATING A GREENHOUSE

STRAWBERRY CLIPPER BEETLE

(About the author: Eric is MOFGA’s Technical Services Director, essentially an organic "extension agent". He can be reached at the MOFGA office to answer your questions about farming and gardening. Link to MOFGA Contact Page, or email Eric directly.)